The invention relates to lasers with unstable optical resonators having totally reflecting optics providing azimuth mode coupling, and in particular to such systems employing high optical gain.
The construction of high power output lasers requires increasing the mode volume. Although this may be accomplished with stable resonators by coupling the output beams of several lasers together into a single output beam, as in U.S. Pat. No. 3,855,544, the approach is complex and expensive and, as in cases where high output power is transmitted through partially transmitting optics, the optics may be destroyed by thermal heating because of absorption thereby of some fraction of the output power. In general, stable resonators do not couple to large volumes efficiently.
Unstable optical resonators are common in the art for providing coupling, with good optical quality, to large volumes as, for example in U.S. Pat. No. 3,824,487. By good optical quality is meant that coupling is achieved with uniform azimuth and radial modes without phase reversal in the output aperture. However, conventional unstable resonators utilize a cylindrical volume of active gain medium and in order to achieve the requisite large volume for high power output as achieved herein one must resort to inordinately long systems. In consequence, the physical characteristics of the optical systems tend to become impractical when high power outputs are attempted with conventional lasers employing unstable resonators.